What took you so long? Was it the perpetuity clause in the release? That always trips them up. I hope you explained that it's for her protection as well.

Shooting a nude portrait is more complicated than it seems — not much more complicated, but more complicated than, say, not doing it at all. I spoke to some nude photographers, as well as some people who photograph nudes, and they had lots of advice for the home pornographer. Dim the lights, they said, and keep the room warm. Use an unfussy, unstained background, put on some classy music (Sade is good), and allow your subject to get suitably, but not prohibitively, wasted. Poses that create long lines — stretched and gracefully twisted limbs and torsos, for example — are more flattering than timid crouching, which tends to cause ugly rolls. Your subject should feel as relaxed and safe as possible, so any lies or mind tricks you can use will help a lot. "It's almost like nudity becomes a kind of clothing, so think of it that way," says Kathleen Rooney, nude model and author of Live Nude Girl: My Life as an Object. "You're not naked, which is vulnerable and accidental. You're nude, which is purposeful and empowered." Yeah, tell her that. Also, be sure to coax the session along with lots of what thoughtful people call "positive affirmations." Photographer Andrew Brucker advises, "Be positive in your speech and sort of encouraging in your exclamations... much like you would to a child." Indeed, shouts of "Nice tits!" and "Wider, wider" can really bolster her confidence and keep that look you describe (resentment, I think it's called) from marring an otherwise perfectly wankable shot.